We examine aspects of professional collaboration in a lesson study conducted with mathematics teachers from the final years of basic education belonging to public schools in the State of Rio Grande do Sul, Brazil. The lesson study, conceived as a professional development approach focused on collaboration and reflection, took place in the 2nd semester of the 2019 school year, organized in 12 meetings of 2.5 hours each, and involved 8 tenured teachers in schools belonging to the 15th Regional Education Coordination of that State. The empirical material of the study consisted of the field notes produced at each meeting of the lesson study, the data from an initial questionnaire administered to the teachers, as well as the transcription of the audio recordings of the interviews with the participants and of the sessions of the lesson study. The qualitative and interpretative analysis showed that the dynamics of the lesson study enhanced several basic aspects of collaboration, namely mutual encouragement and support, cooperation and shared reflection.